This invention relates to systems and techniques involving signal processing in single or multibeam sensing systems, and, more particularly, to systems and techniques involving signal processing of receive signals in single or multibeam sonar systems.
Briefly, by way of background, a sonar system may be used to detect, navigate, track, classify and locate objects in water using sound waves. Defense and civilian applications of sonar systems are numerous. In military applications, underwater sound is used for depth sounding; navigation; ship and submarine detection, ranging, and tracking (passively and actively); underwater communications; guidance and control of torpedoes and other weapons; and mine detection. Most systems are monostatic, but bistatic systems may also be employed.
Civilian applications of underwater sound detection systems are numerous as well. These applications are continuing to increase as attention is focused on the hydrosphere, the ocean bottom, and the sub-bottom. Civilian applications include depth sounding (bathymetry); bottom topographic mapping; object location; underwater beacons (pingers); wave-height measurement; doppler navigation; fish finding; sub-bottom profiling; underwater imaging for inspection purposes; buried-pipeline location; underwater telemetry and control; diver communications; ship handling and docking aid; anti-stranding alert for ships; current flow measurement; and vessel velocity measurement.
A typical active sonar system includes a transmitter (a transducer commonly referred to as a "source" or "projector") to generate the sound waves and a receiver (a transducer commonly referred to as a "hydrophone") to sense and measure the properties of the reflected energy ("echo") including, for example, amplitude an phase. In a typical multibeam sonar system, a first transducer array ("transmitter or projector array") is mounted along the keel of a ship and radiates sound. A second transducer array ("receiver or hydrophone array") is mounted perpendicular to the transmitter array. The receiver array receives the "echoes" of the transmitted sound pulse, i.e., returns of the sound waves generated by the transmitter array. Thus, typically, in a sonar system a short burst of energy is generated by the transmitter array, travels to the target, is reflected, and returns to the receiver array which measures the return signal. A conventional sonar system and transmitter and receiver array configuration is disclosed in Lustig et al., U.S. Pat No. 3,114,631.
An important consideration in the operation of sonar systems is the ability to control, compensate or reduce sources of errors when, for example, employing the system in such applications as depth sounding, bottom topographic mapping, object location, fish finding, sub-bottom profiling, underwater imaging for inspection purposes, and buried-pipeline location. One significant source of error stems from inaccurate sound velocity data. The speed at which sound travels through water changes according to a particular geographic location and time of day. That is, the velocity of sound varies at a given location according to temperature, salinity, and pressure or depth.
Typically in littoral areas, however, the sound velocity profile is a dominate source of error. Littoral areas experience large variations in the velocity of sound which are difficult to measure reduce, or control. Littoral waters tend to include a significant number of "fresh" water outlets/channels and encounter large variations in water temperature. Errors in the sound velocity profile often prevents the use of a very wide swath sonar systems.
Conventional techniques for monitoring the sound velocity profile and minimizing errors created by incorrect sound velocity profile data require a high volume use of velocity profilers and expendable bathythermographs (XBTs). These techniques can be both time consuming and costly as well as presenting logistical concerns. In this regard, a line or array of velocity profilers (thermistors) is coupled to the hull of the ship and hauled during sonar data collection. This manner of sound velocity profile correction limits the velocity at which the ship may travel which, in turn, limits the rate of sonar data collection because the array of velocity profiles should suspend "vertically" in order to provide an accurate correlation between the position of each profiler and its "calculated" absolute depth.
Moreover, employing velocity profilers also presents another disadvantage in that there exists a significant risk in damaging the velocity profiler array as the array is dragged during data collection. In this regard, the velocity profiler array may get damaged on an undiscovered "outcrop" or sea mount as the ship traverses the ocean, for example, during bottom topographic mapping in littoral regions.
Use of XBTs presents an additional concern in that these devices presume that the salinity of the water is known and somewhat fixed. In littoral areas, for example, the salinity may not be known because littoral regions tend to include a large number of fresh water outlets.
The importance of accurate sound velocity data may be illustrated via the following example. The velocity of sound (i.e. velocity of propagation) in sea water typically is within the range of 4700 to 5100 feet per second. In simply measuring the distance to a target, the sound velocity has a significant effect upon the "actual" distance to the target. The time between the instant when the sound leaves and the echo returns (Echo Time) is a measure of distance to the target (i.e., the target range). In general, the distance or range to a target is: EQU Range=1/2*Velocity of Sound*Echo Time
Thus, employing this equation, if the echo time is 10 seconds and the velocity of sound is approximated at 4,800 feet per second, then the distance to the target is 24,000 feet. However, depending upon the temperature, salinity, and pressure or depth of the water, this distance may vary between 23,500 to 25,500 feet. Thus, small changes in the velocity have a significant impact on the operation of a sonar system and the accuracy of the data generated thereby.
Another dominant error source which prevents the use of a very wide swath sonar systems in littoral waters is roll bias error. Roll bias error is the difference in the attitude between the sonar arrays and the hull of the ship. Roll bias error is typically caused by misalignment of the sonar array during installation or by sensor drift. A roll bias error tends to result in, for example, inaccurate bathymetry data.
Presently, vertical reference data source (e.g. multi-axis accelerometer units) are insufficient to provide adequate data to significantly reduce such errors. Although this is expected to change in the near future and improved vertical reference data sources should reduce or correct errors caused by roll bias, such devices are expensive and add to the complexity of the overall sonar system. That is, the next "generation" of multi-axis accelerometer units should improve the considerations for all roll related errors, including roll, heave and pitch biases but the cost and the increase in complexity of implementing such devices in sonar systems may present a concern.
As a result, there exists a need for a sonar system having a signal processing system and technique which compensates, reduces or eliminates, in situ errors in the measured sonar data resulting from inaccurate sound velocity data and/or roll bias error to thereby increase the accuracy in depth sounding, bottom topographic mapping, object location, sub-bottom profiling, underwater imaging for inspection purposes, and buried-pipeline location applications. In particular, there exists a need for a signal processing system and technique to provide an accurate sound velocity profile to compensate, reduce or eliminate the impact of geographic and temporal variations that the velocity of sound have on a sonar system. Further, there exists a need for a sonar system that minimizes bathymetry errors created by incorrect sound velocity profile data which change throughout the day from one locale to another, especially in littoral zones.
Moreover, there exists a need for a signal processing system for a sonar system that compensates, reduces or eliminates roll bias errors resulting from improper alignment of the sonar arrays relative to the ship's hull. There exists a need for a sonar system which minimizes bathymetry errors created by differences in the attitude between the sonar arrays and the hull of the ship which prevent the use of wide swath arrays, especially in littoral zones.